A receiver system generally receives a radio-frequency (RF) signal, and converts the RF signal to an intermediate-frequency (IF) signal via its mixer and filter, which is further processed by post-stage circuit modules. FIG. 1 shows a conventional receiver circuit 10. As shown in FIG. 1, the receiver circuit 10 includes an RF amplifier 11, which has a gain G1 that provides pre-selectivity gain to amplify an RF signal. A mixer 13 converts the RF signal to an IF signal. A filter 15 filters out-of-band signal. An IF amplifier 17 has a gain G2 that provides post-selectivity gain to amplify the IF signal. Generally, the mixer 13 and the filter 15 may have limited linearity and noise performance, thus it is desirable to optimize the strength of signals that are coupled to them.
An input signal received by the receiver circuit generally includes channels at different frequencies. For a specific signal channel, the signal strength of a signal in an adjacent interference channel may be much higher than that of a signal in the desired channel, and the strength of the input signal may also change quickly. Thus, it is desirable that the receiver circuit could fast and automatically adjust its circuit gain in a wide range, so as to output a desired signal stably. Some automatic gain partitioning methods have been used in the receiver circuit 10 shown in FIG. 1.
FIG. 2 shows a conventional receiver circuit 20 with automatic gain control. As shown in FIG. 2, the circuit 20 uses an RF power detector 23 to detect the power of a signal outputted by an RF amplifier 21, and an RF gain controller 25 controls the gain of the RF amplifier 21 according to the detected power of the RF signal. Similarly, an IF power detector 29 detects the power of a signal outputted by the IF amplifier 27, and an IF gain controller 31 controls the gain of the IF amplifier 27 according to the detected power of the IF signal. However, when a high-strength interference signal is present outside the band of the desired signal, the circuit 20 may excessively decrease the gain of the RF amplifier 21 and excessively increase the gain of the IF amplifier 27, and as a result, the signal-to-noise ratio (SNR) of the circuit 20 may drop.
FIG. 3 shows another conventional receiver circuit 40 with automatic gain control. The circuit 40 only has an IF power detector 49 and an IF gain controller 51 coupled to an IF amplifier 47, and controls gain partitioning between an RF amplifier 41 and the IF amplifier 47 according to the detected power of an IF signal outputted by the IF amplifier 47. However, this method may excessively increase the RF gain of the RF amplifier 41, which increases distortion. In certain cases, all of the linearity margin of the system may be partitioned to the RF amplifier 41. The gain of the RF amplifier 41 may be configured to be a minimum gain that satisfies the predefined SNR required by post-stage circuit modules. As a result, the SNR of the output signal of the circuit may be maintained within a predefined range regardless how good the SNR of the input signal is, which constrains the best performance the receiver circuit may achieve.
FIG. 4 shows a further conventional receiver circuit 60 with automatic gain control. The circuit 60 has an RF power detector 63 coupled to an RF amplifier 61 and an IF power detector 69 coupled to an IF amplifier 67, respectively. The circuit 60 also has a gain partitioning controller 71 that controls the gain partitioning between the RF amplifier 61 and the IF amplifier 67 according to the detected power of the RF and IF signals. However, as the speed of detecting the IF power is much slower than that of detecting the RF power, this method leads to a slower response speed of the gain of the RF amplifier 61 to variances of the input signal of the RF amplifier 61. Moreover, if the output signal at the RF amplifier 61 is desired to have a wide adjustable range, the requirement of the dynamic range of the RF power detector is very high, which makes the receiver circuit 60 more complicated and difficult to be implemented.
Thus, there is a need for a receiver circuit to address the aforementioned defects of the conventional receiver circuits.